The present invention relates to an improved rotary hot foil stamping machine and an improved method for hot stamping foil images onto a substrate.
Hot foil stamping is a well-known printing process by which decorative images or text may be transferred or xe2x80x9cstampedxe2x80x9d onto a substrate such as paper. The medium transferred to the substrate may be pigment-based, metal-based, plastic-based or made of other known materials depending on the desired appearance of the final image to be stamped onto the product. Most common foils include five layers. First a polyester film or other carrier provides a flexible protective base on which the remaining layers are supported. Next, a release coat is applied to the carrier layer. The release coat allows the foil pigments and other layers to be separated from the film carrier when the decorative image or text is transferred to the substrate. Below the release coat, a color coat is provided which carries the various pigments which define the color of the transferred image. In metal-based foils, a metal layer, usually aluminum, is then provided to add reflective qualities to the foil image. Finally, an outer adhesive layer is provided to bond the transferred image to the substrate.
A press is provided for transferring the desired foil image onto the substrate. The press acts to stamp or compress, the foil and the substrate together between a heated die and a rigid counter-surface. The heated die includes a raised surface having the form of the image to be applied to the substrate. When the die compresses the foil against the substrate, only the foil area beneath the raised portion of the die is affected. The heat and pressure from the die releases the image-forming layers of the foil from the carrier, and bonds the image to the substrate. Thus, an image corresponding to the raised surface of the die is transferred to the substrate.
Rotary hot foil stamping is a variation of hot foil stamping that is carried out on a rotary press. In a rotary press, the die is cylindrical, with the image portion of the die protruding slightly from the outer circumference of the die. The cylindrical die is mounted adjacent a cylindrical counter roll such that the raised surface of the die engages the outer surface of the counter roll as the die is rotated past it. Both the foil and the product substrate are provided in the form of continuous webs that may be fed between the rotary die and the counter roll. As with other hot foil stamping processes, the die is heated, and when the raised surface of the die compresses the foil web against the product web, the pigments other layers between the raised portion of the die and the counter roll are released from the foil web and bonded to the product substrate. Thus, the foil image is rotationally stamped onto the substrate, as opposed to being vertically stamped as in a platen press.
Rotary hot foil stamping is well-suited for large volume applications in which an image is to be repeatedly stamped along the length of the product web at regular intervals. The product web may then be divided or cut into individual products. Mass mailing pre-printed envelopes, for example, are particularly well-suited for rotary hot foil stamping. Decorative text such as the sender""s name and address or other important information may be consistently and accurately stamped along the length of a large paper roll. The pre-printed roll may then be cut up into envelope blanks which are later folded into envelopes. By properly spacing each foil image along the paper roll, the image will appear in exactly the same location on each envelope after it is cut from the web and folded. Of course the aesthetic advantages of rotary hot foil stamping need not be limited to pre-printed envelopes. The appearance of other products such as brochures and fliers and many others products bearing text or decorating images may also be enhanced through the use of rotary hot foil stamping.
A critical production parameter for the rotary hot foil stamping application is the overall product length on the product web. For example, if envelopes are being stamped, the overall product length must correspond to the length of the unfolded envelope blanks. The product length determines the distance separating each image along the length of the product web so that the images will appear in the same position on each product. This requires the product web to be accurately positioned each time a die surface is rotated past the counter roll such that when the next die surface causes the foil to be compressed against the product web, the product web has traveled exactly one product length since the beginning of the previous stamped image.
Another critical production requirement is that during the actual stamping of an image onto the product, the foil web and the product web must be traveling at the same linear speed as the working surface of the die. If the foil or product webs are traveling at speeds different from the die speed, the stamped image will be distorted, or one or the other of the foil and product webs may be torn. In the past, as a practical matter this has meant that the rotational speed of the die has determined the production rate of the entire rotary hot foil stamping process.
Prior art rotary stamping machines rely on the physical size of the rotary die to accurately position images along the product web. The product and foil webs are fed at the same speed as the surface of the die throughout the process. The die may have a single image or multiple images formed around circumference of the die. In order for images to be stamped the proper distance apart, the circumferential distance between adjacent die surfaces must equal the desired distance between images stamped onto the product web. The circumferential distance between image surfaces around the outer circumference of the die is a function of the die""s diameter. Therefore, products requiring larger distances between images (corresponding to longer product lengths) require larger diameter dies, whereas short product lengths require smaller diameter dies. In other words, in prior art rotary hot foil stamping machines, the product length is dependent on, and established by, the diameter of the rotary die. Each die must be custom fit for a particular application, with the die circumference being dictated by the product length. Custom made rotary dies are expensive, and the expense has generally had a limiting effect on the use of rotary hot foil stamping, restricting its use to those applications having sufficiently large volumes to justify the production cost of a custom die.
It should be noted, however, that in most applications the foil image is to occupy only a short segment of the overall product length. Therefore, the actual stamping surface of the die is compressed against the foil and product webs for only a brief portion of the entire rotation of the die. At all other times a gap exists between the surface of the rotary die and the counter roll such that both the foil web and the product web can move freely therebetween. Observing this, the present inventors have discovered an improved rotary hot foil stamping machine wherein the foil web and the product web are fed independently of the rotary die and the counter roll. In such a machine, both the foil web and the product web must be accurately and synchronously positioned with respect to the rotating die such that a foil image is repetitively stamped in the proper location on each ensuing product. Further, the foil and product webs must be controlled such that both travel at the same speed as the surface of the die during the actual stamping of the image onto the product web. By removing the correlation between the diameter of the rotating die and the position of the foil and product webs, it becomes possible to provide a standard sized rotating die or a die holder on which dies may be removably attached to form different stamped images, and different product lengths may be selected independently of the diameter of the die or die holder and dies. It also becomes possible to control the foil web independently of the product web so that less foil is wasted between stamped images, thereby increasing the efficiency of the rotary hot foil stamping process.
The present invention provides an improved rotary hot foil stamping machine, and an improved method for repeatedly hot stamping a foil image along the length of a continuous product web.
According to a preferred embodiment of the invention, the improved rotary hot foil stamping machine includes a die station comprising a rotary die or die holder and a counter roll. In an especially preferred embodiment a rotary die holder is provided which is configured to receive one or more detachable die segments which may be mounted to the outer circumference of the die holder. The product web and a foil web are fed between the die or die holder and the counter roll such that as the die or die holder is rotated, the die surfaces protruding therefrom attached thereto are rotated against the counter roll. As a die surface is rotated past the counter roll, it compresses, or stamps, the foil against the product web and against the counter roll. The die is heated so that the stamping of the foil against the product web releases a foil image from the foil web, and bonds the image to the surface of the product web.
Servo driven feed rolls and pull-through rolls are located respectively on the input and output sides of the die station. The feed rolls and pull-through rolls may be rotated at highly accurate speeds and stopped at precise positions after rotating a preset angular distance, to very accurately position the product and foil webs relative to the rotating die or die holder, which is also driven by a precision servomotor. As the die or die holder rotates, the feed rolls and pull through rolls feed the product web and the foil web through the die station. When the foil image is being stamped the feed rolls and pull-through rolls advance the product and foil webs at feed rates equal to the speed of the rotating die surface. When the stamping surface of the die is rotated clear of the counter roll, however, the product web feed rate is changed to a second product web feed rate so that the product web will be repositioned to the proper location when the next image is stamped onto the product web. The foil web may be slowed between image stampings, to conserve the foil web, or, if a second die station is provided as described below, the foil web feed rate may be increased between image stampings to supply a sufficient length of foil web so that both the first and second die stations are supplied with adequate amounts of foil to simultaneously stamp images at two locations along the length of the product web.
The various feed rates, and the overall position of the product and foil webs relative to the die station, are controlled based on the angular position of the die holder. By synchronizing the feeding of the product and foil webs with the angular position of the die holder, and by altering the product web feed rate during the intervals between image stampings, the distance between stamped images along the length of the product web may be made independent of the diameter of the die holder. Thus, custom dies, or die holders, need not be made each time it is desired to produce a product having a different product length. Instead, all that is required is that new removable die segments be produced incorporating the new desired image and attached to the rotating die holder. The new product length and the new image length must then be entered into the servo control system to adjust the feed rates and feed lengths to match that of the new product. Thus, the cost of producing a new hot foil rotary stamped product is greatly reduced.
Another feature of the present invention is that a second die station may be added to effectively double the speed at which products are conventionally produced. The second die station is substantially identical to the first die station, and is rotated synchronously therewith. Thus, the second die station stamps images simultaneously with the first die station, and, as with the first die station, the product and foil webs may be fed at accelerated feed rates in the intervals between stampings. With a second die station, an additional product length must be fed in the interval between stampings such that both the first and second die stations will be stamping clear areas of the product web that have not been previously stamped. Additional die stations may also be added to further increase production rates, however practical limitations as to how fast the product and foil webs may be fed, and cost factors related to building equipment that can achieve such speeds may limit the efficacy of adding further additional die stations.
The present invention further relates to an improved method for repeatedly hot stamping a foil image onto a product web at regular fixed intervals. The method involves providing a rotary die holder, and mounting at least one rotary die thereto, and providing a counter roll adjacent to and spaced apart from the rotary die holder such that the surface of the die engages the surface of the counter roll as the die is rotated past it. The method further comprises the step of rotating the die holder at a constant speed so that the surface of the die is rotated against the counter roll at regular timed intervals. The next step involves feeding a product web and a foil web between the counter roll and the die holder. The product web and foil web are fed at a first feed rate during the period of the die holder""s angular rotation when the surface of the die engages the counter roll, the first feed rate corresponding to the linear velocity of the surface of the rotating die. The product web is fed at a second feed rate in the intervals when the die stamping surface is not in contact with counter roll so that the product web is in the proper location to receive the next stamped image as the next die surface is rotated against the counter roll.
The method may be further refined by providing a second die holder and counter roll, and rotating the second die or die holder synchronously with the first die or die holder, in a manner. similar to the second die station of the improved rotary hot foil stamping machine described above. Upon adding a second die station, the second product feed rate must be adjusted to feed a sufficient length of product web during the intervals between stampings to present clear unstamped product lengths to both the first and second die holders so that two products may be stamped simultaneously. The foil web feed rate must also be adjusted during the between stamping intervals so that sufficient foil is available at each die station to form both sets of images on the product web. By providing the second die station, the production rate of the rotary hot foil stamping process may be effectively doubled.